The stock of the world’s fourth-largest plane manufacturer was already on a tear. Tuesday’s recent gains of 2.8% lifted Embraer’s New York-traded shares to $37.76, for a year-to-date ascent of 32%, nearly double the S&P 500’s stellar performance and a huge contrast to the 8% decline in Brazil’s key stock index.

Is it time to sell?

Citigroup thinks so, pointing out that the order is “likely to be heavily discounted” and that “almost all of Embraer’s EJet order flow this year has come from just two carriers operating in the same market.”

Embraer’s deal with SkyWest adds to two others this year, one with United Airlines Inc. (UAL) and another with Republic Airways.

SkyWest has firm orders for 40 E175 regional jets at list prices that total $1.6 billion. Deliveries are to begin in the second quarter of next year and wrap up by mid-2015. It will operate the planes for United Airlines.

Sixty more Embraer jets can be reconfirmed if SkyWest reaches deals with other US carrier partners. It also has options to buy another 100 jets from Embraer.

Moreover, Citi notes real risks to the order book, including “excessive discounting” on commercial jet orders, not to mention greater competition in the business and commercial jet segments and possible migration of customers to bigger and longer-range aircraft. Coupled with the stock’s valuation and potential for “erosion in backlog quality,” and Citi has a share price target of $30, a 21% nose-dive from its current level.